''The Christians never wanted us to survive,'' he said. ''They dominated the state machinery and kept our villages poor. That's why my family had to come to Beirut, to the shantytown of Karantina. But the Christians couldn't bear to have us so near them, and destroyed it.

''I lost two brothers in those battles,'' he continued. ''But I'm still fighting. My aim is to do away with the Christian dictatorship here.''

Much has changed in Lebanon in the years since that Christian and that Muslim spoke of war. Then, Christians and Syrians, Muslims and Palestinians, were locked in a civil war that had bankrupted the power of the Lebanese government. Today, there is relative peace in this splintered nation. The bulk of Palestinian fighters have been evacuated to other Arab countries. In a surprising show of unity between Christians and Muslims, they have elected a new Lebanese president. The Lebanese Army is slowly reasserting itself - in Beirut, at least - after seven years of virtual impotence.

On a tightrope somewhere between the embattled points of view of the Maronite monk and the street fighter is the compromise between Christians and Muslims that held Lebanon together from independence in 1943 until the 1975-76 civil war. (There was an earlier outbreak of fighting in 1958.)

Now, the youthful new President, Amin Gemayel, is committed to trying to put that compromise back together again for Lebanon's estimated 3 million citizens. In an interview conducted shortly before the dramatic sequence of events that led to his election, he stressed his commitment to Lebanon's existing constitutional formulas.

''This does not mean we are calling for freezing the situation exactly as it is,'' he added, ''but that we propose change within the framework of respect for the basic texts of our constitutional entity.''

Many of Lebanon's problems stem from its being geographically at the focus of regional rivalries. But outside forces, Arab or Israeli, would never have been able to get a foothold inside Lebanon if it had not had serious internal problems. And its major internal problem is demography.

Within its present boundaries, Lebanon has only existed as an administrative entity since 1920 - a short period of time in this region of ancient East Mediterranean cultures. Before that, it was part of the Ottoman Empire. In 1920, the French, who governed the area of present-day Syria and Lebanon under a mandate from the League of Nations, split the area: The southwestern chunk they called the State of Greater Lebanon; the other part approximated what is now Syria. Greater Lebanon included the craggy slopes of the historical Mt. Lebanon, site of the biblical cedar forests. In the mountain area, some degree of self-government had been in existence, on and off, for nearly three centuries.

But the new state also included the plains north, south, and east of the mountain, as well as the coastal cities of Tyre, Sidon, Tripoli, and Beirut. Under the Ottoman Empire, these areas had belonged to other administrative blocks, and had had little in common with Mt. Lebanon.

Thus, by the single stroke of a French pen, the communities of these outlying areas were thrown into the new state along with the traditional communities of the mountain. The population mosaic that resulted looked something like this:

* At its heart lived the two traditionally peasant groups of the mountain. The Maronite Christians, one of the largest Eastern-rite communities of the Roman Catholic Church, had come to the northern peaks of the Lebanon range (most probably from northern Syria) sometime before the 10th century AD. Over the succeeding centuries they had gradually spread themselves farther south.

* The Maronites' partners - and sometimes antagonists - in the traditional Mt. Lebanon administration were the Druzes, members of an esoteric quasi-Muslim sect that took root in the southern part of the mountain before the end of the 11th century AD. Faced with persecution and infiltration from more orthodox Muslims, the Druzes decided early on to forbid conversion to their religion, and to keep their doctrines secret. Druzes believe their number was determined at the time of the Creation, and that when a Druze dies his soul enters the body of a newborn Druze.

Kamal Salibi, a Lebanese historian, has described the communities that developed in Mt. Lebanon as ''rebel mountaineers, hardy and clannish, with a staunch particularism and a strong spirit of independence. . . . (They are) industrious peasantry, conventionally generous and hospitable, naively cunning in their political and social dealings, and passionately fond of heroics.''

* Around the mountain lived the peasants of the plains and steppes. To the south and east these were mainly Shiite Muslims (from whom the Druzes were originally converted).

For two centuries, up until 1171 AD, Shiism had been the dominant sect throughout the East Mediterranean. But in that year, the Shiite ruler in Cairo was ousted by the (orthodox Muslim) Sunnite general, Saladin. Without protection from Cairo, most of the Shiite communities of the East Mediterranean, including those of the Akkar plain north of Mt. Lebanon, converted to Sunnism. But those of the Bekaa Valley east of Mt. Lebanon, and those of Mt. Jebel Amil to its south, were remote enough to cling to their Shiite doctrine.

From 1171 on, the Muslim empires of the region, whether based in Cairo, Baghdad, or finally Istanbul (the Ottomans), were all officially Sunnite. The population mix of Lebanon's coastal cities became similar to those of other key trading centers throughout the Ottoman Empire, with many family and community links between them - links that have continued up to modern times. Most of the coastal cities (except Tyre) were composed of Sunnites, along with large groups of Christian minorities tolerated by the Empire.

Most of these Christians belonged to the Greek Orthodox Church. The Orthodox, who have never sought to tie themselves to the West, speak Arabic and have produced some of the region's foremost Arab-nationalist thinkers.

(In 1724, some of these Orthodox congregations switched their allegiance to Rome and became known as Greek Catholics.)

Since the creation of Greater Lebanon in 1920, further fugitive population groups have sought refuge in and near its port cities. These have included the Armenians, whose population in Lebanon numbered over 350,000 members at its peak; about 500,000 Palestinians; and thousands of Kurds and Assyrians. Among these refugee groups, however, only the Christians - the Armenians, the Assyrians, and a few of of the Palestinians - have been granted Lebanese citizenship.

Between 1920 and 1943, the French administration tried various formulas for welding the unwieldy creation into an effective, unified state. It was not very successful. The people of the outlying areas and the coast still sought to continue their traditional links with neighboring Syria, rather than look to the new administration. And the French were determined that Lebanon be a Christian state, despite the fact that, even then, the Muslims came close to outnumbering the Christians - which they have since done.The breakthrough came in 1943, and was reached by the Lebanese themselves. Two gifted politicians, the Maronite Bishara Khuri and the Sunnite Riyadh as-Sulh, agreed to the following unwritten formula, since then known as the National Pact:

* The Christians would no longer seek French or other Western patronage.

* The Muslims would no longer seek union with Syria.

* Maronite fears of eventual Muslim domination would be allayed by allotting the presidency and the command of the army to Maronites.

* The premier would be a Sunnite, and other top state posts would be divided among members of other sects.

* Seats in the parliament would be divided between Christians and Muslims on a 6-to-5 basis.

After 1943, this National Pact became an established and successful fact. Both sides were, to a large extent, weaned away from their tendency to look for outside support. The open-door economic liberalism of the regime kept money flowing in from more tightly controlled Arab neighbors. Apart from the clashes in 1958, the system seemed stable enough to most observers.

But the demographic balance in the country was changing all the time, as consistently higher birthrates boosted the Muslim population. During the same period, the poles of outside attraction for the various communities were changing, too. By the early 1970s, many Muslims were looking to the presence of the armed Palestinian guerrillas to help strengthen their cause. And by mid-1976 , it was clear that Israel (which had not even existed in 1943) had become a major influence on some of the Maronites.

The question facing President Gemayel, as he starts his six-year term, is whether the ''Spirit of '43'' can be revived to unite all Lebanon's sects against outside interference.

One substantial asset he had at the outset was the support of most traditional Muslim leaders. But then, the Muslim position today is much weaker than it was in early 1976, when only the intervention of Syria saved the right-wing Christian militias from total defeat at their hands.

Now the Muslim and allied militias in south and central Lebanon have been routed by the Israelis, and the Palestinian fighters evacuated from Beirut (at the request of the Muslim leaders themselves). Strong right-wing Christian militias, backed by the Israeli Army, want undiluted power and are clamoring for the end of Palestinian and Syrian presence in Lebanon. And for Muslim partisans still ''bearing arms'' in northern and eastern Lebanon, the traditional alternative of seeking integration into Syria holds little attraction under the present regime there.

In short, the Muslims may be able, in late 1982, to gain the same real influence in government they obtained in 1943.

''But some sort of consensus is certainly necessary,'' points out Georgetown University's Michael Hudson, an expert on modern Lebanese politics, ''if only because Lebanon is a country that cannot easily be ruled by coercion alone.''

Already, President Gemayel has stressed his commitment to rejoining the Christian and Muslim halves of Beirut. But will Christian students from East Beirut feel safe crossing back to the colleges and universities of the west? Will Muslim businessmen from the west feel safe crossing to the port in the east? Can the polyglot restaurants, universities, mountain resorts, beach clubs, libraries, cinemas, and theaters of Lebanon return to the vitality they enjoyed until 1975?

Lebanon as a political entity will need serious care and attention in the coming months. But Lebanon as a cultural and business center can probably be fairly quickly rebuilt, given the resilience the Lebanese have already exhibited after each of their successive misfortunes over the past few years - provided that the country's traditional links to the Arab world remain.